Frequency modulation distance determining system



Nov. 4, 1947. w. R. MERCER 2,430,357

FREQUENCY MODULATION DISTANCE DETERMINING' SYSTEM Filed March 2, 1944'Fggfz E 7%@ FEEQ 3196K :L Smcnfor Patented Nov. 4, 1947 FREQUENCYMDULATION DISTANCE DETERMINING SYSTEM William R. Mercer, Hightstown, N.J., assgnor to Radio Corporation of America, a corporation f DelawareApplication March 2, 1944, Serial No. 524,793

6 Claims.

This invention relates to radio distance measuring systems and moreparticularly to improvements in the art of measuring distances byreflection of frequency modulated signals. F-M distance measuringsystems in general are well known to those skilled in the art, beingdescribed in Bentley Patent 2,011,392 and Espenscheid Patent 2,045,071.In these systems a radio wave that is radiated to a reflecting surfaceor obj ect is cyclically frequency modulated. In a receiver which islocated close to the point of radiation, the reflected signal is pickedup and mixed or heterodyned with some of the frequency modulated signalreceived directly from the transmitter (the point of radiation). Theresulting difference frequency is a measure of the distance from thetransmitter to the reflecting surface or object, since this frequency isdetermined by the time required for the radiated signal to reach thereflecting object and return to the receiver.

The principal object of the-present invention is to provide an improvedmethod of and means for minimizing the effects of extraneous signals andrandom reflections upon the operation of systems of the described type.

Another object is to provide an improved method of and means forcontrolling the amplication of the difference frequency signal inaccordance with the strength of the reflected signal or the frequency ofthe beat signal, or both, so as to provide discrimination againstundesired signal components and noise.

These and other objects will become apparent to those skilled in the artupon consideration of the following description, with reference to theaccompanying drawing, of which Figure 1 is a schematic block diagram ofa F-M distance measuring system,

Figure 2 is a group of curves illustrating variations in the frequencycharacteristic as a function of the output amplitude of one of theelements of the system of Figure 1,

Figure 3 is a schematic circuit diagram off an amplifier providing thecharacteristics illustrated in Figure 2, and

Figure 4 is a group of curves showing the eiects of various componentsof the circuit of Figure 3.

Referring to Figure 1, a radio transmitter I is connected to an antenna3 and to a frequency modulator 5. The modulator 5 may be of the typedescribed in U. S. application Serial No. 471,003, filed January 1,1943, by Sydney V. Perry, and entitled Capacity modulator unit. Areceiving antenna 'I is connected to a balanced detector circuit 9 whichmay be of the type described in U. S. application Serial No. 445,720,led June 4, 1942, by R. C. Sanders, Jr., and entitled Frequencymodulated altimeter or distance indicator, or which may be any type ofbeat frequency detector. The detector 9 is also connected to thetransmitter I'. The output circuit of the detector 9 is connected to anamplifier II, described in more detail hereinafter. The output circuitof the amplifier II is connected through a limiter circuit I3 to afrequency responsive circuit I5 such as an averaging cycle counter ofthe described type in U. S. Patent 2,228,367, issued Janilary 14, 1941,to R. C. Sanders, Jr., and entitled Frequency meter. The output circuitof the counter I5 is connected to an indicator I'l such as a D.C. meteror the like.

The amplifier II is provided with automatic gain control means includinga rectifier I9 connected to its output circuit to provide a D.C. controlvoltage for application to a grid bias circuit of the amplier I`I. Thegain vs. frequency characteristic of the amplifier II at low signallevels, when the output of the rectifier I9 is substantially zero, isillustrated by the curve 2I of Figure 2. The gain increasessubstantially linearly (in db. per octave) with increase in frequency upto a frequency ,f corresponding to the maximum distance which theequipment is to measure. Beyond this point the gain decreases sharplywith increase in frequency. The curves 23, 25 and 27 illustratesuccessive gain vs. frequency characteristics provided by the amplifierII as its output increases, increasing the bias voltage supplied by therectier I9. With increase in output, the amplifier gain decreasesrelatively rapidly over the range of frequency f to f, and decreasesrelatively slowly over the range of frequency up to f.

At frequencies above f, signals are all affected as if the circuit weresimply an ordinary AVC. A signal in the region of f first causes adecrease in high frequency gain as shown by the successive curves 2|y 23and 25 thus discriminating against any signal of higher frequency thanitself. Then as the amplitude becomes still greater, the signal itselfis held from overloading the amplier by AVC action.

A study of the first curve will show that the amplitude at transitionfrom automatic response control to AVC action becomes progressively 10W-er as the frequency goes up, approaching immediate AVC at the frequencyf of peak again.

In the operation of the system of Figure 1, the transmitter I produces aradio frequency output which is cyclically varied in frequency by themodulator 5. This output is radiated from the antenna 3 to a reflectingobject 29 to which the distance is to be measured. Part of the energyradiated by the antenna 3 is reflected by the object 29 to the receivingantenna 1, where it is picked up and applied to the balanced detector 9.Energy is also applied to the detector 9 directly from the transmitter Ithrough the connections 3i. The output of the detector 9 includes a beatfrequency Voltage having a frequency equal to the instantaneousdifference in frequency between the transmitted and received signals.Since the received signal is delayed with respect to the transmittedsignal by the time required for the energy to travel from the antenna 3to the reiiector E9 and back to the receiving antenna 1, the frequencyis proportional to the distance of the reflecting object 29 from theantennas.

rThe beat frequency output of the detector 9 is amplified by theamplifier II, limited to a consta-nt amplitude by the limiter I3 andapplied to the counter circuit i5, which provides a D.C. outputproportional in magnitude to the beat frequency. The output of thecounter I is applied to the meter i1, deflecting it in accordance withthe distance to be indicated. When the distance of the reflecting object29 is relatively great, the frequency of the beat signal is high and theamplitude is low owing to the usual attenuation of radio waves withdistance. EXtraneous signals or interference of relatively highamplitude and low frequency, if allowed to pass through the amplifier li, may actuate the limiter I3 and the counter l5, causing the indicatorI 1 to provide an erratic or incorrect indication. However, as indicatedby the curve 2i of Figure 2, the gain of the amplifier Il under theseconditions is relatively low at low frequencies, providingdiscrimination against undesired low frequency signal components.

As the distance to the reflecting object 29 is decreased, the frequencyof the'beat signal decreases and its amplitude increases causing anincrease in the amplitude of the output of the amplifier I l, bothbecause of less distance attenuation and more gain until the point ofpeak gain is passed. The input to the amplier Il, however, may containextraneous higher frequency components resulting, for example, fromreiiections from a more distant reflecting object.

Under some conditions, these undesired higher frequency signa-1components may attain amplitudes of the same order as that of thedesired relatively low frequency signal. These components tend to causeerratic operation of the counter I5, and incorrect or erratic indicationby the meter I1, particularly if the gain of the amplifier I l increaseswith increase in frequency. This eect is minimized or eliminated by therelatively rapid decrease in gain of the amplifier I I over the higherfrequency range with increase in output amplitude, as indicated by thecurves 23, 25 and 2i.

Referring to Figure 3, a preferred circuit arrangement for the amplifierII includes an electron discharge tube 33 of the pentode voltageampliiier type such as that commercially designated as GSJ'?. The tube33 is provided with the usual anode load resistor 35 and a self-biasresistor 3i, connected in the cathode circuit. Resistors 39 and 5I,connected between the positive anode supply terminal B-I- and ground,function as a voltage divider providing screen grid potential for thetube 33. The control grid of the tube '33 is connectedh to the outputcircuit of the detector 9, and through a network comprising resistors 43and `I5 and capacitors 41 and 49 to the anode of the tube 33. The anodeof the tube 33 is coupled through a capacitor 5i and a low pass filternetwork comprising resistors 53 and 55 and capacitor 51 to the followingstages (not shown) of the amplifier II. A resistor 52 is connected fromthe capacitor 5I to ground.

The rectifier circuit I9 includes a tube 59 with itsV cathode circuitcoupled through a capacitor 5l to the output circuit of the amplifier II, The cathode of the tube 59 is also connected through a resistor S3and a battery 65 to ground. The anode of the tube 59 is connected to aload resistor 6,1 shunted by capacitor B9, and through a filtercomprising series resistors 1l and 13 and a shunt capacitor l5 to thecontrol grid of the tube 33.

The network comprising the resistors i3 and 55 and capacitors 41 and 49,together with the output impedance of the detector 9, provides feedbackfrom the anode to the grid of the tube 33. The constants of this networkare such as to provide a minimum negative feedback, or slightlypositiveV feedback, in the region of the upper frequency limit andnegative feedback over the lower frequency range. The gain provided bythe tube 33, with the feedback network, is illustrated by the curve BIof Figure 4. As the bias applied to the tube 33 is increased, the gainchanges as indicated by the successive curves 83, 85, and 81 in Figure4. The action is that of ordinary automatic Volume controlY operating inthe less degenerated frequency range.

The resistor 52 and capacitor 5I are proportioned to cut the lowfrequency response, providing a transmission vs. frequencycharacteristic illustrated by the dash curve 89.

The low pass iilter section comprisingV resistors 53 and 55 andcapacitor 51, in conjunction with the input capacitance of the followingamplifier stage, is designed to attenuate principally at frequenciesabove the frequency f, as illustrated by the dotted curve SI. Thiseliminates the sharp peaking ofthe overall gain curve at f and providesa relatively sharp cutoff above thisifrequency. The combined effects ofthe transmission Vcharacteristics represented by the curves 89 and 9IYare superimposed upon the gain characteristics of curves 8-I, 83, 85 and81` to provide the overall` gain vs. frequency curves 2-I, 23, 25 and21, respectively (Figure 2) The operation of the system of" Figure 3 isas follows: When the amplitude of the input, and hencethat of the outputof the amplifier is low, the gain vs. frequency characteristicV issubstantial1y as illustrated by the curve 2l of Figure 2; This conditioncontinues as long as the ampliiier output peaks do not exceed thevoltage of the battery 65 and the tube 59 remains non-conducti-ve. Asthe outputV increases, the tube 59 becomes conductive during thenegativev half cycles,

allowing current to iioW through the resistor 61' and charging thecapacitor 69 negatively with respect to ground potential'. The storageaction of the capacitor 69 together with the low pass characteristic ofthe filter'1I, 13; 15` smooths the output ofthe tube 59 to a D.C.voltage which increases inY magnitude with increase in amplitudeY J" tof and more particularly in the zero or positive feedback region around,'f the gain decreases more rapidly with increase in bias, causing theshape of the gain vs. frequency characteristic to change as indicated inFigure 2.

I claim as my invention:

1. In a radio distance measuring system including means for producing avoltage having a frequency related to the distance to be ascertained, anamplifier including input and output circuits, a feedback circuitincluding frequency selective means connected from said output circuitto said input circuit, rectifier means connected to said output circuitof said amplifier and means for controlling the gain of said ampliierrespons'to the output of said rectifier means.

2. The invention as set forth in claim 1 wherein said frequencyselective means comprises impedance elements proportioned so as toprovide negative feedback varying with frequency over the lowerfrequency portion of the range of operation of said amplifier, andpositive feedback in the region of the upper limit of the frequencyrange of operation of said amplifier.

3. The invention as set forth in claim 1 wherein said amplifier includescoupling elements so proportioned as to provide a rising characteristicof gain vs. frequency.

4. The invention as set forth in claim 1 including means for biassingsaid rectier circuit to prevent operation thereof except when theamplitude of the input thereto exceeds a predetermined value.

5. In a radio distance determining system including means for producinga, voltage having frequency proportional to the distance to beascertained, means for amplifying said voltage, said amplifier meanshaving normally a transmission characteristic directly proportional tofrequency, and means connected to said amplifier to automaticallydecrease the gain thereof at frequencies in the region of the upperlimit of the transmission range, in accordance with a predeterminedfunction of the amplitude of the output of said amplifier means.

6. In a radio distance measuring system including means for transmittinga frequency modulated signal, receiving said signal after reflection,and combining said transmitted and received signals to provide a beatsignal, an amplifier including an electron discharge tube provided withan anode, a cathode, and a control grid, means for applying said beatsignal to said control grid, a feedback circuit connected from saidanode to said control grid and including frequency selective meanswhereby the phase shift and the attenuation of the signal fed backthrough said circuit are predetermined functions of the frequency ofsaid beat signal, a rectifier circuit coupled to the output circuit ofsaid amplier, and means for applying the output of said rectifiercircuit to said control grid to control the bias thereon as apredetermined function of the amplitude of the output of said amplifier.

WILLIAM R. MERCER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,280,109 Varela Apr, 21, 19422,206,903 Lane et a1 July 9, 1940 2,268,587 Guanella Jan. 6, 1942

